Bovine viral diarrhea virus (BVDV) is a small enveloped positive-stranded RNA virus and is classified, along with hog cholera virus (HOCV) and sheep border disease virus (BDV), in the pestivirus genus. Pestiviruses were previously classified in the Togaviridae family (Fenner, 1975), but studies on their genomic organization (Collett, et al., 1988b,c) allowed their reclassification within the Flaviviridae family along with the flavivirus and hepatitis C virus (HCV) groups (Francki, et al., 1991). However, more recent studies on genomic organization, virion composition, and genomic RNA structure (Thiel, et al., 1991; Brock, et al., 1992) suggest, instead, a distinct family for pestiviruses.
BVDV is a worldwide, economically important pathogen of cattle and can be distinguished, based on cell culture analysis, into cytopathogenic (CP) and noncytopathogenic (NCP) biotypes (Gillespie, et al., 1960). The NCP biotype is more widespread although both biotypes can be found in cattle (Moennig and Plagemann, 1992). If a pregnant cow becomes infected with an NCP strain, she can give birth to a persistently infected (PI) and specifically immunotolerant calf that will spread virus during its lifetime (Roeder and Harkness, 1986). The PI individual can succumb to mucosal disease (MD) and both biotypes can then be isolated from the animal (Brownlie, et al., 1984). Since these biotypes are antigenically similar, it is probable that the NCP strain goes through mutations to give rise to a CP strain instead of a superinfection with the latter (Howard, et al., 1987; Corapi, et al., 1988). Other clinical manifestations can include abortion, teratogenesis, and respiratory problems (Radostits and Littlejohns, 1988). Apart from MD, cattle infected with BVDV generally develop a mild diarrhea followed by the appearance of neutralizing antibodies with a rapid recovery (Radostits and Littlejohns, 1988). In addition, severe thrombocytopenia, associated with herd epidemics, that may result in the death of the animal (Rebhun, et al., 1989; Corapi, et al., 1989, 1990) has been described and strains associated with this disease seem more virulent than the classical BVDVs (Bolin and Ridpath, 1992).
Several economically-devastating BVDV outbreaks among both veal calves and older animals were recently observed in Canada. In 1993, the mortality rate among veal calves in Quebec increased four times and was estimated at 31.5% for grain-fed calves and 17.1% for milk-fed calves (overall mortality was 32,000 out of 143,000 calves; Dr. G. Rivard, Ministere de l'Agriculture, des Pecheries et de I'Alimentation du Quebec (MAPAQ), Quebec, Quebec, Canada).